1. Field of the Invention
The present invention relates to a thermal printer unit used in POS, ECR, barcode printing, a measuring instrument and the like, and particularly to a technology which can improve print quality.
2. Description of the Related Art
The thermal printer unit is a mechanism constituting a main part of a thermal printer. As is well known, in the thermal printer unit, a positional relationship between a thermal head and a platen roller has a large influence on the print quality. Therefore, the positional relationship between a thermal head and a platen roller is very important, and thus an adjusting mechanism or a mechanism, which obtains a proper relationship even if the adjustment is not performed, is generally provided in the thermal printer unit.
However, when receptor paper (paper) is conveyed by the platen roller, disadvantageously a position of the thermal head moves in a paper conveyance direction with respect to the platen roller. Therefore, there is known a technique of preventing the thermal head from moving in the paper conveyance direction (for example, see Japanese Patent No. 2762045). In the technique, a part of a member to which the thermal head is attached engages a shaft of the platen roller to prevent the thermal head from moving in the paper conveyance direction.
There are following problems in the thermal head movement preventing method disclosed in Japanese Patent No. 2762045. As shown in FIG. 14, a force F1 is applied to a thermal head T by a force F2 in a rotating direction, when thermal recording paper or receptor paper W is fed in a horizontal direction while the thermal recording paper or thermal transfer ribbon and the receptor paper W are pressurized and nipped between the thermal head T and a vertex of the platen roller. However, in order to obtain the high print quality, sometimes the thermal recording paper or the thermal transfer ribbon and the receptor paper are pressurized and nipped at a position where the thermal head T and the platen roller are located away from the vertex of the platen roller. A force F3 in the rotating direction is applied to the thermal head T when the thermal recording paper or the receptor paper W is fed from below as shown in FIG. 14. Therefore, the thermal head T moves in the rotating direction to change a relative position between the thermal head T and the platen roller E, which possibly results in a decrease in image quality. Furthermore, there has been the following problem in the thermal head movement preventing method. That is, the force F3 in the rotating direction which is applied to the thermal head from the receptor paper depends on a thickness and stiffness of the paper, when the sheets of receptor paper are differ from each other in type such as a tag (for example, thickness of 160 μm and stiffness of 78 mm) and a label (thickness of 150 μm and stiffness of 39 mm). Therefore, an amount of change in relative position initially set between the thermal head T and the platen roller E varies among the sheets of receptor paper having the different thickness and stiffness. When the printing is performed to the thermal recording paper or the receptor paper, in order to keep the optimum relative position between the thermal head T and the platen roller, it is necessary that position be adjusted by a position adjusting mechanism such that the relative position becomes the optimum between the thermal head T and the platen roller E in consideration of the amount of change in the relative position. It is also necessary that position be set again in each type of the thermal recording paper or receptor paper.
Additionally, sometimes a driving portion of the platen roller is located only on one of sides, or sometimes a distortion phenomenon is generated on both sides of the thermal head due to a driving delay or a pressurizing balance.